In a semiconductor laser it is quite important in its application to make the laser oscillate at as short as possible wavelength.
Furthermore, laser elements which emit a plurality of different wavelength laser light beam are quite useful in such as optical communications, and there are conventionally developed a variety of types of apparatus.
FIG. 1 is a cross-sectional view showing a prior art quantum well type semiconductor laser shown in such as Appl. Phys. Lett. vol. 39, pp. 134 to 137 (1981). In FIG. 1, the reference numeral 1 designates an n.sup.+ type GaAs substrate, the reference numeral 2 designates an n type Al.sub.z Ga.sub.1-z As cladding layer, the reference numeral 3 designates an n type Al.sub.z Ga.sub.1-z As parabola type refractive index distribution layer (zgradually changes to y), the reference numeral 4 designates a p type Al.sub.x Ga.sub.1-x As quantum well active layer, the reference numeral 5 designates a p type Al.sub.y Ga.sub.1-y As parabola type refractive index distribution layer (y gradually changes to z), the reference numeral 6 designates a p type Al.sub.z Ga.sub.1-z As cladding layer, and the reference numeral 7 designates a p.sup.+ type GaAs contact layer.
Furthermore, FIG. 2 is a cross-sectional view showing a prior art quantum well type semiconductor laser recited in such as Appl. Phys. Lett. vol. 45, pp. 1 to 3 (1984). In FIG. 2, the reference numeral 8 designates a p type electrode, the reference numeral 9 designates a p type GaAs substrate, the reference numeral 10 designates a p type Ga.sub.0.65 Al.sub.0.35 As cladding layer, the reference numeral 11 designates a p type Ga.sub.0,7 Al.sub.0.3 As light guide layer, the reference numeral 12 designates a multiple quantum well layer (MQW layer). The reference numeral 13 designates an n type Ga.sub.0.65 Al.sub.0.35 As cladding layer, the reference numeral 14 designates an n type GaAs contact layer, the reference numeral 15 designates a Zn diffusion region, the reference numeral 16 designates a disordered region, the reference numeral 17 designates a silicon dioxide film, and the reference numeral 18 designates an n type electrode.
FIG. 3 shows a prior art short wavelength semiconductor laser recited in Iwamura et al., Japanese Journal of Applied Physics vol. 24, No. 12, December 1985, pp. L911 to L913. The reference numeral 19 designates an n type electrode, the reference numeral 20 designates an n.sup.+ type GaAs substrate, the reference numeral 21 designates an n type AlGaAs cladding layer, the reference numeral 22 designates an n type AlGaAs confinement layer, the reference numeral 23 designates an AlGaAs/AlGaAs multi-layer quantum well active layer, the reference numeral 24 designates a p type AlGaAs confinement layer, the reference numeral 25 designates a p type AlGaAs cladding layer, the reference numeral 26 designates a p.sup.+ type GaAs contact layer, and the reference numeral 27 designates a p type electrode.
Herein, the n type AlGaAs cladding layer 22 has an aluminum composition proportion lower than that of the n type AlGaAs cladding layer 21, and the p type AlGaAs cladding layer 24 has an aluminum composition proportion lower than that of the p type AlGaAs cladding layer 25, and thus a SCH structure is produced.
A semiconductor laser in which the active layer of the usual double heterostructure laser is replaced by a quantum well structure or a superlattice structure is called a quantum well type semiconductor laser. A laser which includes a potential well layer in the active layer is called a single quantum well laser, and a laser which includes a plurality of potential well layers in the active layer is called as a multi-quantum well laser.
The operation of the semiconductor laser element having such a quantum well structure active layer will be described below.
FIG. 4 is a diagram showing the energy level in the quantum well structure. The longitudinal axis designates energy, and the transverse axis designates position perpendicular to the layers. As shown in FIG. 4, when a quantum well is constituted by putting a thin semiconductor layer between semiconductor barrier layer shaving a large energy band gap, then this thin semiconductor layer constitutes a potential well layer, and the intrinsic energy En measured from the bottom (top) of the conduction (valence) band of the electrons (holes) confined in this well is represented by the following Schrodinger equation (1), and forms discrete energy levels. ##EQU1##
Herein, m.sub.e * is an effective mass of electron, h is a Planck'w constant divided by 2.pi., and Lz is thickness of the quantum well layer. Herein, the thickness and height of the barrier layer are presumed to be infinity so as to enable one dimensional treating.
In this way an electron has quantized energies En, and the electron state density .rho.(E) becomes to have a step like distribution as shown by the real line in the quantum well, which electron state density usually has a parabola type distribution as shown in broken lines in the bulk crystal.
Accordingly, when both of the layers above and below the active layer comprising the quantum well layer are made a p type semiconductor cladding layer and an n type semiconductor cladding layer having large energy band gaps, respectively, it is possible to confine the carriers (electrons or holes) and the light in the thickness direction, which enables the production of a quantum well type semiconductor laser. A semiconductor laser constructed in this manner has an n=1 level (the lowest quantum level) higher than the bottom of the conduction band (real lines in FIG. 5) when the active layer is produced by the material which has a same energy band gap as that of the material constituting the active layer of the usual double heterostructure semiconductor laser, and therefore it oscillates at a shorter wavelength because the energy difference is larger than that of a usual double heterostructure semiconductor laser which oscillates by the energy difference between the bottom of the conduction band and the top of the valence band. Furthermore, the quantum well type semiconductor laser has discrete energy level characteristics as well as a narrow spectrum line width and a good monochromaticity.
The prior art device of FIG. 1 will be described.
First, an n type Al.sub.z Ga.sub.1-z As layer which will be a cladding layer 2 will be grown on the n.sup.+ type GaAs substrate 1, and subsequently, an n type Al.sub.z Ga.sub.1-z As (z gradually changes to y) parabola type refractive index distribution layer 3, a p type Al.sub.x Ga.sub.1-x As quantum well active layer 4, a p type Al.sub.y Ga.sub.1-y As (y gradually changes to z) parabola type refractive index distribution layer 5, a p type Al.sub.z Ga.sub.1-z As cladding layer 6, and a p.sup.30 GaAs contact layer 7 will be grown (herein, z&gt;y&gt;x). FIG. 6 shows an energy band structure of the cladding layer, the refractive index distribution layer, and the active layer of the semiconductor laser produced in this manner.
Herein, above and below the active layer 4, the larger the aluminum composition proportion is the farther from the active layer, and the lower the refractive index is, whereby the light generated at the active layer 4 is confined within this parabola type refractive index distribution layer. Furthermore, a cavity is produced with the end facet, whereby a laser oscillation is enabled to occur. This type of laser is called as a GRIN-SCH (Graded-index waveguide separate carrier and optical confinement heterostructure) laser because of the structure in which the confinement of the carriers and light are separated.
In this prior art device, when carriers (electrons of holes) are injected biased in a forward direction, carriers are confined in the quantum well active layer 4, and the electrons and holes are recombined at discrete energy levels (n=1, n=2, . . . ) of the conduction band and the valence band, thereby to generate a light emission. In this place, a sharp light generation wavelength peak (.lambda..sub.1, .lambda..sub.2, .lambda..sub.3, . . . ) in accordance with the energy levels can be obtained. In a general laser the gain at n=1 becomes larger than the resonator loss, and an oscillation occurs at the quantum level of n=1. However, when the loss is increased, combinations of carriers not only at between the n=1 quantum energy levels of the conduction band and the valence band but also those between the quantum energy levels n=2 thereof become likely to arise as shown in FIG. 4, thereby producing a laser oscillation at the quantum level of n=2. The oscillation wavelength of n=2 is fairly short relative to the oscillation wavelength of n=1 because the difference between the quantum levels n=1 and n=2 is fairly large.
Next the operation of the prior art example of FIG. 2 will be described.
FIG. 7 shows a diagram for explaining the energy band structure of the cladding layer and the multi-quantum well active layer of the prior art device of FIG. 2. In this prior art device having a MQW active layer, if the barrier layer is made quite thin, there arises overlapping of energy levels of the potential wells in the active layer, and it is possible to make almost all the electrons positioned at the lowest energy levels of a few quantum wells.
From this, in a MQW laser, the half value width of the optical gain spectrum becomes short relative to the usual DH laser caused by the stepwise state density function .rho.(E) as shown in FIG. 5, and the threshold value is expected to be lowered because of the higher density of states.
When a current is injected into this MQW laser which is biased in a forward direction, the electrons injected from the upper portion n electrode 18 and the positive holes injected from the lower portion p electrode 8 are confined in the GaAs well layer having a small band gap, and electrons and holes are combined to produce light emission. Light confinement in the thickness direction is accomplished by making the aluminum components in the AlGaAs layer larger as the refractive index becomes smaller, and the confinement in the transverse direction is conducted by diffusion of Zn. Accordingly, if a resonator structure is produced in addition thereto a laser oscillation will arise.
Next, the operation of the prior art device of FIG. 3 will be described.
The gain spectrum of the active layer obtained when a current is applied in the prior art device of FIG. 3 becomes as shown by the real lines of FIG. 8. Herein, n=1, 2, and 3 designate the peaks corresponding to the lowest, second lowest, and third lowest quantum levels, respectively. When a current is injected the peak corresponding to n=1 has the largest gain and a laser oscillation occurs at the wavelength corresponding to this quantum level.
In a prior art semiconductor laser of such a construction, it is required to make the quantum well layer quite thin or to increase the aluminum composition proportion in order to conduct an oscillation at a short wavelength. However, it is quite difficult to control the thickness of the quantum well layer to be quite thin, and furthermore when the aluminum composition proportion is increased end surface oxidation is likely to arise thereby to shorten the useful life of the lesser. Furthermore, when the quantum well is made thin and the aluminum composition proportion is increased the threshold current becomes high. These are obstacles to shortening the wavelength by these methods.
Furthermore in the prior art semiconductor laser with such a construction, the energy level at which the laser oscillates is controlled. Generally, an oscillation occurs at an energy level of n=1, and an oscillation at a short wavelength of n=2 is not likely to occur. In a case where an oscillation occurs at the quantum level of n=2 when the injection current is increased as described in the operation of the prior art device of FIG. 4, the oscillation wavelength difference corresponding to the energy difference .DELTA.E.sub.12 between the quantum energy levels of n=1 and n=2 becomes quite large. For example, in a case of GaAs quantum well type active layer of layer thickness 100 .ANG., .DELTA.E.sub.12 is about 110 meV and the wavelength difference is 570 .ANG..
The prior art semiconductor laser which emits a plurality of laser light beams will be described.
FIG. 9 shows a laser structure of a transverse junction stripe laser which emits four kinds of different wavelength light which is recited in such as Appl. Phys. Lett. vol. 36, p. 441 (1980). In FIG. 9, the reference numeral 28 designates an upper portion electrode, the reference numeral 29 designates a silicon nitride current blocking layer, the reference numeral 30 designates an n type Al.sub.y Ga.sub.1-y As cladding layer, the reference numeral 31 designates an n type Al.sub.x1 Ga.sub.1-x1 As first active layer, the reference numeral 32 designates an n type Al.sub.x2 Ga.sub.1-x2 As second active layer, the reference numeral 33 designates an n type Al.sub.x3 Ga.sub.1-x3 As third active layer, the reference numeral 34 designates an n type Al.sub.x4 Ga.sub.1-x4 As fourth active layer, the reference numeral 35 designates an n type GaAs substrate, the reference numeral 36 designates a lower portion electrode, the reference numeral 37 designates a Zn diffusion region which is shown by diagonal lines, and the reference numeral 38 designates a Zn driven region which is shown by broken lines.
Herein, the aluminum composition proportions of the cladding layer and active layer are in a relation that x.sub.1 &lt;x.sub.2 &lt;x.sub.3 &lt;x.sub.4 &lt;y.
FIG. 10 shows an array in which three lasers are integrated on a substrate which is recited in such as Appl. Phys. Lett. vol. 48, p. 7 (1986). In FIG. 10, the same reference numerals designate the same elements as those shown in FIG. 9. The reference numeral 30 designates a p type GaAs contact layer, the reference numeral 40 designates a p type AlGaAs cladding layer, the reference numeral 41 designates an AlGaAs/GaAs multi-layer quantum well active layer, the reference numeral 42 designates an n type AlGaAs cladding layer, the reference numeral 43 designates a Si diffusion layer (shown by diagonal lines), and the reference numeral 44 designates a proton irradiated region (shown by dotted lines).
The device will be operated as follows.
In FIG. 9 the holes which flow from the p side upper electrode 28 and the electrons which flow from the n side lower electrode 36 proceed along the path designated by the arrows in the figure, and they generate light emissions at the four active layers 31, 32, 33, and 34. Herein, wavelengths of the generated light beams are such that .lambda..sub.1 &lt;.lambda..sub.2 &lt;.lambda..sub.3 &lt;.lambda..sub.4 because x.sub.1 &lt;x.sub.2 &lt;x.sub.3 &lt;x.sub.4 (.lambda..sub.n designates the wavelength of light which is emitted from the n-th active layer).
In FIG. 10, holes which flow from the upper electrode 28 and electrons which flow from the lower electrode 36 are recombined at the three active layers 41 which are not diffused by silicon, thereby to occur an oscillation.
The prior art multi-wavelength oscillation laser can be produced only in a transverse junction stripe type which is shown in FIG. 9. This method has a problem in cost because the process thereof is a complicated one. Furthermore, such a prior art multi-wavelength oscillation laser is one which emits a plurality of wavelengths at the same time, and it is impossible to produce an oscillation with switching one by one. Furthermore, in the laser array shown in FIG. 10 the three active layers emit the same wavelength light, and it is impossible to conduct multiplexing of different wavelength light.
FIG. 11(a) and (b) shows a construction of a prior art wavelength multiplexed optical communication integrated semiconductor laser which is recited in Appl. Phys. Lett. vol. 29, p. 506 (1976).
In FIG. 11, the reference numeral 45 designates an n electrode, the reference numeral 46 designates a p electrode, the reference numeral 47 designates an n type GaAs substrate, the reference numeral 48 designates n type Al.sub.0.3 Ga.sub.0.7 As, the reference numeral 49 designates Zn diffusion front, the reference numeral 50 designates p type GaAs, the reference numeral 51 designates p type Al.sub.0.2 Ga.sub.0.8 As, the reference numeral 52 designates a p type Al.sub.0.07 Ga.sub.0.93 As, and the reference numeral 53 designates a periodic concavo-convex. Furthermore, the reference numeral 54 designates a laser, the reference numeral 55 designates a waveguide, the reference numeral 56 designates a coupler, and the reference numeral 57 designates an optical fiber.
This laser produces oscillation caused by distribution feedback being conducted by the periodical concavo-convex. When the period of the concavo-convex is made .LAMBDA., the oscillation wavelength .lambda. becomes as in the following. EQU .lambda.=2n.LAMBDA./m (2)
Herein, n designates an effective refractive index of the waveguide, and m designates an integer. The .LAMBDA. is quite small, and the production of the concavo-convex is difficult. Accordingly, m=3 is usually used.
As shown in FIG. 11(b), when .LAMBDA. is changed slightly in several waveguides 55 light beams of different wavelengths are obtained from the formula (2). However, in a case of producing such an integrated laser, producing concavo-convex which have slightly different .LAMBDA. on the respective waveguides is difficult and the concavo-convex are likely to be destroyed due to the second epitaxial growth. This results in a bad yield and difficulty in production, thereby leading to a high cost.